170 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS IIETEOROLOGY. 



tions. He has discovered tliat near tlie parallel of 5° N. in tlie At- 

 lantic Ocean the pressure of the'atmosphere is so uniform as to afford 

 navigators a natural standard by which, out there at sea, they may, 

 as they pass to and fro, compare their barometers. This pressure 

 is said to be so uniform, that after allowing for the six-hom'ly fluc- 

 tuations, the mariner may detect any error in his barometer 

 amounting to the two or three thousandth part of an inch. 



365. According to the ^dews presented in § 358 and Plate YII., 

 Enrio'moSm-?'^' ^'^^ south-cast trade-muds, which reach the shores 

 SfreSonso'f^the''''' ^^ ^^'^^^^ ^^^1' ^^® parallel of Kio, and which blow 

 northern hemisphere, thouce for the most part ovor the land, should be 

 the winds which, in the general com^se of circulation, would be 

 carried, after crossing the Andes and rising up in the belt of 

 equatorial calms, towards Northern Africa, Spain, and the South 

 of Europe. They might carry with them the infusoria of Ehren- 

 berg (§ 358), but according to this theory, they would be want- 

 ing in moistm^e. Now, are not those portions of the Old World, 

 for the most part dry countries, receiving but a small amount of 

 precipitation ? Hence the general rule : those countries to the 

 north of the calms of Cancer, which have large bodies of land 

 situated to the southward and westward of them, in the south-east 

 trade-wind region of the earth, should have a scanty supply of rain, 

 and vice versa. Let us try this rule : The extra-tropical part of 

 New Holland comj^rises a portion of land thus situated in the 

 southern hemisphere. Tropical India is to the northward and 

 westward of it ; and tropical India is in the north-east trade- wind 

 region, and should give extra-tropical New Holland a slender 

 supply of rain. But what modifications the monsoons of the 

 Indian Ocean may make to this rule, or what effect they may have 

 upon the rains in New Holland, my investigations in that part of 

 the ocean have not been carried far enough for final decision ; 

 though New Holland is a dry country. 



366. The earth is nearer to the sun in the summer of the 

 Each hemisphere re- southcm hemisphere than it is in the summer of the 

 thrsamc'amounrof northcm ; Consequently, it has been held that one 

 ^^^'- hemisphere annually receives more heat than the 

 other. But the northern summer is 7.7 days longer than the 

 southern ; and Su' John Herschel has sho^^n, ^.nd any one who wiU 

 take the trouble may demonstrate, that the total amount of direct 

 solar heat received annually by each hemisphere is identically the 

 same, and therefore the northern hemisphere in its longer summer 



